1. Field of the Invention
The present invention relates to voltage-controlled quartz crystal oscillators (VCXO) and, more specifically, to an integrated circuit for modulating, with a voltage-controlled phase modulation loop, the frequency of a quartz crystal oscillator.
2. Discussion of the Related Art
There are several solutions to modulate the frequency of a voltage-controlled quartz crystal oscillator.
FIGS. 1 and 2 show two conventional examples of voltage-controlled quartz crystal oscillators, based on structures of Colpitts type. FIG. 1 shows a Colpitts-type oscillator in which the frequency is modulated, discreetly, by switching capacitors in series with the quartz crystal. FIG. 2 shows an example of Colpitts-type oscillator in which the frequency is modulated, linearly, by means of a variable-capacitance diode (varicap).
A Colpitts-type oscillator is, for example, formed of an NPN-type oscillation transistor To, the collector of which is connected to a first terminal A of positive supply Vcc, and the emitter of which is connected to the midpoint B of a series association of two capacitors Cbe and Ce between the base of transistor To and a second supply terminal M, generally connected to the ground. A first terminal of a quartz crystal 1 is connected to the base of transistor To which forms a terminal S of output of the oscillating signal. When a modulation of the oscillation frequency is desired, a second terminal of the quartz crystal is connected to terminal M via a capacitive modulation circuit, respectively 2 (FIG. 1) or 2' (FIG. 2). Biasing resistors Rb, Re, are generally provided between terminal S and terminal A and between the emitter of transistor To and terminal M. A Colpitts-type oscillator can also be implemented with other types of oscillators (PNP, JFET, . . . ).
The operation of a Colpitts-type oscillator is well known. For the assembly to oscillate, two conditions must be met. A first condition is that the resistance seen from quartz crystal 1 has to be, in absolute value, higher than its own series resistance. A second condition is that the resistance seen from the quartz crystal has to be negative. The function of transistor To and of capacitors Cbe and Ce is precisely to create a negative resistance between terminals B and M. The oscillation frequency is determined by the equivalent capacitance of the parallel assembly on quartz crystal 1.
The assembly shown in FIG. 1 is more specifically meant for frequency shift keying (FSK).
Frequency modulation circuit 2 is formed, for example, by two capacitors C1, C2, interposed, in parallel, between quartz crystal 1 and terminal M. Each capacitor C1, C2, is respectively connected, in series, with a diode D1, D2, the cathodes of which are connected to terminal M. The anode of each diode D1, D2, is connected to the corresponding capacitor C1, C2 and, via a resistor R1, R2, to a terminal 3, 4, used to apply a control voltage V3, V4. Diodes D1 and D2 are diodes with a series impedance which is very high in their nonconductive state and very low in their on-state. When one of voltages V3, V4, is positive enough, the corresponding capacitor C1, C2, contributes, by looping back through the ground, to the capacitance present in parallel on quartz crystal 1. Thus, the frequency modulation is performed by the application of a voltage on one of terminals 3 and 4 or on both terminals.
The assembly of FIG. 2 is more specifically meant for linear modulation (FM).
Modulation circuit 2' is formed by a varicap diode Cv interposed between quartz crystal 1 and terminal M. The anode of varicap diode Cv is connected to terminal M and its cathode is connected to a terminal of quartz crystal 1 and, via a resistor R3, to a terminal 5 used to apply an oscillator control voltage V5. The frequency modulation is performed by varying voltage V5 which modifies the equivalent capacitance of varicap diode Cv.
A disadvantage of conventional Colpitts-type assemblies is that they require a relatively large area for integration and some portions are generally not integrated. Indeed, modulation circuits 2 (FIG. 1) or 2' (FIG. 2) must, in practice, be implemented in the form of circuits external to an integrated circuit including the Colpitts-type assembly itself. In particular, the implementation of a varicap diode in integrated form is very bulky.
To implement a voltage-controlled quartz oscillator in the form of a circuit where all components (except for the quartz crystal) are integrated, another type of assembly, an example of which is illustrated by FIG. 3, is generally used. The principle of such an assembly is to use a variable phase-shifter 6 on a feedback loop connecting an output S of a differential amplifier 7 to the positive input voltage of this amplifier 7. Quartz crystal 1 is connected between the negative input terminal of amplifier 7 and the ground. Variable phase-shifter 6 introduces a delay in the feedback loop. The length of this delay is controlled by a voltage V6 applied on a control terminal of phase-shifter 6. The modulated signal is taken from the output S of amplifier 7.
In such an assembly, amplifier 7 and phase-shifter 6 can be implemented in the form of an integrated circuit. However, the modulation range is much narrower than in a Colpitts-type oscillator.
FIG. 4 illustrates the impedance-frequency characteristic of a quartz crystal. A quartz crystal has two oscillation frequencies. A first fixed frequency fs corresponds to the series resonance of the quartz crystal and is a function of the intrinsic characteristics of the quartz crystal, that is, its series inductance and its series capacitance. At frequency fs, the impedance Z of the quartz crystal corresponds to its series resistance. A second oscillation frequency fp of the quartz crystal corresponds to the parallel frequency of the quartz crystal and is variable. Frequency fp is a function of the series inductance intrinsic to the quartz crystal and of the capacitance in parallel on this series inductance. Frequency fp is thus variable according to the capacitances added in parallel across the quartz crystal. At frequency fp, the quartz crystal impedance is at its maximum.
An assembly such as shown in FIG. 3 uses the series resistance of the quartz crystal, since the quartz crystal impedance must be minimum for the negative input of amplifier 7 to be substantially at the ground, as otherwise the oscillation is damped. Now, when the frequency is varied around frequency fs by means of phase-shifter 6, the quartz crystal impedance increases. It must thus be seen to it that frequency modulation range .DELTA.f is sufficiently low for the quartz crystal impedance to remain sufficiently low, so that the assembly oscillates without damping.
U.S. Pat. No. 3,728,645 discloses a quartz crystal oscillator wherein an active circuit modifies the oscillator frequency. The active circuit introduces a variable capacitance between the base of the oscillating transistor and the ground. A drawback of such a circuit is that its control necessitates a dynamic modulation and that the oscillation frequency cannot be controlled from a D.C. signal.
Another disadvantage of an assembly such as shown in FIG. 3 is that the amplitude of the modulated signal is not stable due to the quartz crystal impedance variation. Further, the central frequency (fs) of the oscillator is not settable.
Oscillators which use an odd number of series inverters, connected in parallel with a quartz crystal, having both its terminals grounded via capacitors, are also known. A switch short-circuits an even number of inverters to modify the propagation time. Such oscillators are exclusively meant for an FSK modulation.